My weekend project was installing a KBD75 PCB into my CLSm.
Why you ask? I am a sucker for QMK (I put a Clueboard PCB into my LZ St), and the KBD75 PCB supports it. My next goal is to look at porting one of my Leeku PCBs to QMK, but my understanding is that the way RGB is supported may make it a little trickier.
The KBD 75 PCB does not have the USB connector in a compatible location so you'll have to do some surgery:
1. You'll have to de-solder the USB connector from the KBD75 to stop it interfering with the acrylic spacer. You may or may not have to dremel the "bump" that extends from the PCB where the USB connector is mounted depending on whether your PCB sits entirely below the top plate edge, which depends on how flush you managed to get your switches to sit.
2. There are USB connection points on the PCB (next to the USB connector) that are a lot easier to solder to than the tiny holes the USB connector is mounted to.
3. I used the adafruit mini usb breakout board. If I was to do it again I'd put a connector between the breakout board and the PCB to make it possible to disconnect the two.
4. I had to dremel the mounting holes and some extra PCB off the breakout board so it would fit in the cutout for the USB connector in the bottom case. On the plus side the breakout board is now braced against the case, so was able to get away with just electrical taping it in place (with a layer of tape around the breakout board itself)
5. The KBD 75 PCB has a couple less LED's than the PCB that comes with the CLSm, and on mine at least one of the LEDs was slightly defective, so it appears to be of lesser quality to me.
But I am now a very happy camper since all of my boards support QMK.